Stop rendering to back buffer

Stop rendering directly to back buffer from qt widget. Use an intermediate fbo and blit the color buffer to Qt afterwards. Hardware stereoscopic 3D mode needs some adjustment, as GL_BACK_LEFT / GL_BACK_RIGHT won't be available anymore. An fbo with 2 color buffers has to be used in that case.

in QtGLGraphicsScene::drawBackground there is a QPainter with native-painting mode used. Maybe the intermediate framebuffer should be bound here instead.